Presence control on BF Princeton Reverb

[SteveJones]

Building an amp using the BF Princeton Reverb circuit (aa1164 circuit). Planning on adding a presence control. For any others that have done the same, was wondering what value pot and cap they ended up with once they were happy with it?

Right now planning on a 25K lin pot and .1 mfd. cap.

Thanks for any advice,

Steve

[Bruce / Mission Amps]

Building an amp using the BF Princeton Reverb circuit (aa1164 circuit). Planning on adding a presence control. For any others that have done the same, was wondering what value pot and cap they ended up with once they were happy with it?

Right now planning on a 25K lin pot and .1 mfd. cap.

Thanks for any advice,

Steve

Steve my only comment would be, why? These amps do not really suffer from loss of presence.
Are you sure you might not like a cross cut "High Cut" control instead?

[SteveJones]

Open to suggestions

Well...

In doing some of Google-ing on this topic, found some folks that had put a presence control on this circuit and enjoyed it (although can't remember where that was). I tend to like extra high end, since my ears are burned out around 5K. Also a presence control adds a bit of grit to the sound, taking out some of the NFB. So thought it would be a good idea...

Also, this amp will have a 12" speaker, so should have a slightly deeper tone than a regular BFPR.

However, this is a build I want to get right the first time and not do a bunch of "redo". So if a presence control is a bad idea, I'm not interested in adding it. So, anyone out there in Ampage Land add this to their amp and were they happy with the results?

Steve

[Chuck H]

Something you need to consider is that the knee frequency of the cap in the presence control is effected by the impedance of the NFB loop. I've often seen the standard "Marshall" value of .1uf used with circuits of different impedance, but the Princeton Reverb's NFB loop impedance is VERY different. It's like that value is just etched into the minds of designers for some reason. So if you want to get it right the first time you need to determine what frequency you want the control to effect and choose a cap value accordingly based on the impedance of the NFB loop.

The Princeton Reverb NFB loop has a MUCH lower impedance than the typical Marshall. A .1 cap in tandem with a 25k pot will only effect frequencies above 10Khz and the pot will only function on the first 10th of it's rotation. To get the same effect as the presence control in a Marshall amp on a Princeton Reverb you would need to use about a 6.8uf cap (obviously this would have to be a non polar electrolytic) and a 250 ohm pot (hard to find, maybe a 500 ohm would work OK).

I wouldn't mess around with the loop values to raise loop impedance on the Princeton Reverb because the raising the 47 ohm shunt resistor will drastically effect the bias of the triode in that circuit.

HTH

Chuck

[SteveJones]

Clarification

Let me clarify. When I say "get it right first time", I don't mind changing values of caps, resistors, pots, etc. But I do mind starting out with a presence control in my design, which influences how I make my front panel, how many holes I drill, and how I lay out all the controls on the panel, etc. and then later changing my mind to remove it from the circuit which kinda wrecks the appearance of the amp. So from the point of planning, it is important for me to decide whether or not to even have this control in the first place. Wish I had kept the links to the pages that led me to think doing this was a good idea...

So in addition to comments regarding the values of the components, I'm also looking for opinions as to whether or not to have this control in the first place from anyone that has done it.

Thanks Chuck for your comments regarding NFB loop impedance and component values. What I don't want to do is take a good circuit and screw it up just to have a particular feature! Based on your comments and the post from Bruce, maybe I should just leave it out.

[Chuck H]

If your going to have a faceplate made you may be able to get a second faceplate (with the "presence" indication and hole omitted) for cheap. Not much retooling involved. Then you could try it in the circuit and if you don't like it just use the 'no presence control' faceplate and don't drill the hole. If your having the chassis punched for you the 'no presence control' faceplate will still cover the hole.

Or, try it in the circuit before drilling and if you like it mount it on the back panel, where a hole is less visible if you decide to remove it later. Don't tell people it's there. They'll wonder how you get your tone and you won't tell them, 'I dunno, I just built this amp...'

Chuck

[Bruce / Mission Amps]

Well...

In doing some of Google-ing on this topic, found some folks that had put a presence control on this circuit and enjoyed it (although can't remember where that was). I tend to like extra high end, since my ears are burned out around 5K. Also a presence control adds a bit of grit to the sound, taking out some of the NFB. So thought it would be a good idea...

Also, this amp will have a 12" speaker, so should have a slightly deeper tone than a regular BFPR.

However, this is a build I want to get right the first time and not do a bunch of "redo". So if a presence control is a bad idea, I'm not interested in adding it. So, anyone out there in Ampage Land add this to their amp and were they happy with the results?

Steve

OK, here's another tweak you can do:
build it stock but use a 68 to 100 ohm resistor under the 1500 ohm biasing resistor. The additional resistance will not alter the bias point enough to make any difference.
Remove the 22uF-25uF cathode bypass cap around that 1k5 resistor in the NFB loop and tweak the stage starting with a .47uF to .68uF cap.
Moving up larger in uF value from there will start bringing in the mids, then low mids, then high lows, then mid lows, then lows and bass... etc etc...
You'll eventually find a tone you like.
If you keep that speaker, I bet you'd rarely touch the NFB loop again.

[jbrew73]

what is the purpose of raising the the 47 ohm to 68-100 ohm? just curious.

[Bruce / Mission Amps]

what is the purpose of raising the the 47 ohm to 68-100 ohm? just curious.

It gives you a little more negative feedback voltage to work with and shunt around the cathode biasing resistor to the cathode to change the tone.... you can also increase the value of the resistor from the OT to reduce the NFB voltage for a looser sound.
There are a couple things going on there with the cathode resistor being lifted from ground and the NFB fed to the junction of the two.